1. Field of the Invention
The present invention relates to In-Band-On-Channel (IBOC) broadcasting receivers, and more particularly, to IBOC broadcasting receivers which receive broadcasting from an IBOC broadcasting station that broadcasts analog sideband and digital information allocated in outer frequency bands of the analog sideband with the same broadcasting frequencies.
2. Description of the Related Arts
IBOC broadcasting that broadcasts analog sidebands and digital information allocated in outer frequency bands of the analog sidebands with the same broadcasting frequencies is known. The IBOC broadcasting is generally divided into two broadcasting modes. One of them is a full-digital broadcasting mode that transmits only digital broadcasting, and the other is a hybrid broadcasting mode that transmits analog broadcasting and digital broadcasting coexisting together. In the IBOC hybrid broadcasting, conventional broadcasting receivers can receive analog broadcast portions of the IBOC hybrid broadcasting.
In the IBOC hybrid broadcasting, an IBOC broadcasting station simultaneously transmits analog broadcasting and digital broadcasting. In this instance, the hybrid broadcasting may be conducted in a simultaneous broadcasting format in which identical broadcasting contents are transmitted in analog broadcasting and digital broadcasting. By using the simultaneous broadcasting format, a blending function to switch receptions between analog broadcasting and digital broadcasting depending on broadcasting receiving conditions can be realized. Bit error rates in the digital broadcasting are used as the basis for judging as to when to switch receptions between analog broadcasting and digital broadcasting by the blending function.
However, in the case of mobile reception which may take place with on-vehicle tuners, signal receiving conditions may substantially change. For example, when a vehicle with an on-vehicle tuner moves through areas where buildings in different heights are densely located, signal receiving conditions may substantially change due to the buildings. In this case, receptions of analog broadcasting and digital broadcasting may be frequently switched from one to the other by the blending function. However, the frequent switching of the formats of broadcasting would psychologically stress listeners who receive the broadcasting. Occurrence of such an incident should preferably be avoided.
As a method to switch broadcasting formats on the side of a broadcasting receiver according to conditions of broadcasting reception, a broadcasting reception method used in the hierarchical transmission system that is conducted in digital BS broadcasting is known. When switching classes in the hierarchy according to this method, two fixed threshold values are used to switch from a higher class to a lower class and from a lower class to a higher class, thereby providing the hierarchical switching with hysteresis.
The switching method described above is effective when substantial changes in broadcasting reception conditions are not frequently observed in a short period of time. Since changes in the weather that may result in switching of broadcasting formats would not occur in a short period of time, the above-described method used in the digital BS broadcasting to switch broadcasting can prevent chattering due to such switching caused by changes in the weather.
On the other hand, when mobile reception is conducted with an IBOC broadcasting receiver, substantial changes in reception conditions can be expected to occur in a short period of time, and the above-described switching method alone cannot prevent chattering due to switching of broadcasting formats.